EP-A-317 340 discloses a Coriolis-type mass flow sensor which can be installed in a conduit and through which a fluid to be measured flows during operation, comprising
a vibrating system containing PA1 a support, particularly a support tube, to which the vibrating system is attached at the ends via bellows and by which the vibrating system is connected with the conduit, and PA1 at least one sensor for the inlet-side vibrations of the measuring tube and at least one sensor for the outlet-side vibrations of the measuring tube. PA1 a single straight measuring tube which is traversed by the fluid and PA1 a support, particularly a support tube, to which the measuring tube is attached at the ends and by which the measuring tube is connected with the conduit, and PA1 at least one sensor for the inlet-side vibrations of the measuring tube and at least one sensor for the outlet-side vibrations of the measuring tube. PA1 a vibrating system containing PA1 a support, particularly a support tube, to which the vibrating system is attached at the ends and by which the vibrating system is connected with the conduit; and PA1 at least one sensor for the inlet-side vibrations of the measuring tube and at least one sensor for the outlet-side vibrations of the measuring tube,
a single straight measuring tube which is traversed by the fluid, and PA2 two node masses extending along the measuring tube and attached to the latter at the ends, the weight of the node masses being so distributed PA2 the vibrating system being excited during operation of the mass flow sensor into stringlike vibration by means of at least one exciter, PA2 which during operation of the mass flow sensor is so excited in one of its natural frequencies of vibration by means of at least one exciter PA2 a single straight measuring tube which is traversed by the fluid and acts as a main vibrator, and PA2 an auxiliary vibrator which does not come in contact with the fluid and PA2 the vibrating system being so excited during operation of the mass flow sensor in one of its natural frequencies of vibration by means of at least one exciter PA2 the vibrating system being so designed PA2 the connecting elements being so designed and arranged
that their center of gravity is located at the center of the axis of the measuring tube, PA3 that it vibrates in a hoop mode, PA3 which is mechanically coupled with the measuring tube via connecting elements, PA3 that the measuring tube vibrates in a hoop mode; PA3 that its kinetic energy is at least twice as high as the kinetic energy of the main vibrator, and PA3 that hoop modes of the measuring tube caused by Coriolis forces are transmitted to the auxiliary vibrator as little as possible.
This mass flow sensor cannot be fully dynamically balanced for all fluid densities, nor is it of a particular compact design, i.e., nor does it have as short an overall length as possible.
In EP-A-316 908 (=U.S. Pat. No. 4,949,583) and in an article by H. Hagenmeyer et al., "Design of an Advanced Coriolis Mass Flowmeter Using the Hoop Mode", published in the proceedings of "FLOMEKO '94, 7th International Conference on Flow Measurement", Glasgow, June, 1994, a Coriolis-type mass flow sensor is described which can be installed in a conduit and through which a fluid to be measured flows during operation, comprising
Investigations have shown, however, that in the case of nonideal fluids, particularly nonhomogeneous fluids, multicomponent fluids, high-viscosity fluids, and highly compressible fluids, such mass flow sensors, in which the measuring tube vibrates in a hoop mode, have a considerably greater measurement error, and therefore are much more inaccurate, than mass flow sensors of the first-mentioned type with a measuring tube vibrating in the manner of string.
This is due to the fact, inter alia, that the main vibrator, consisting of the measuring tube with the fluid to be measured, loses vibrational energy in an irreversible manner, e.g., through sound radiation to the environment. The amount of energy dE/E lost per cycle is inversely proportional to the quality factor Q of the vibrating system in the actual vibration mode: EQU (dE/E).sub.cycle .about.1/Q. (1)